1. Field of the Invention
The present invention relates to a process for reducing the concentration of water soluble metals in aqueous solutions, and, in particular, to a process for removing iron and/or manganese ions from aqueous solutions such as mine discharge waters or spent leaching waters prior to discharging such waters into surface or ground water systems.
2. Description of the Prior Art
The quality of water in the United States is important for the health and the quality of life of the people who live therein. For more than a hundred years the quality of water has been reduced by industrial pollution and deep mining, and, more recently, surface mining. Deep and surface mining pollution of water systems have been especially prevalent in the coal-producing regions of the Appalachian mountains. The pollutants resulting from these types of mining operations include iron, sulfate, manganese, aluminum, and acidity, to name a few. The toxicity of these pollutants impact both health and recreation in those regions.
A current method that is used to reduce the ionic and acidic pollution from the water systems around mining operations consists of raising the pH of the water above 8.5 with caustic soda. Ions then precipitate from the solution as hydroxides and a high pH supernatant is then discharged into streams. This process requires continuous maintenance and replenishment of caustic soda.
Recently, microorganisms have been used to reduce the concentration of pollutants from both industrial wastes and mine discharges. There have been many reports that bacteria can be used to remove metals as sulfides and sulfates from industrial and mining wastes.
It has been shown that sulfate can be removed using sulfate reducing bacteria in an anaerobic system (see U.S. Pat. No. 4,124,501 by Yen et at. and U.S. Pat. No. 4,200,523 by Balmat). Using similar methodology, sulfate reducing bacteria grown under anaerobic conditions has been shown to yield hydrogen sulfide gas which precipitates out metals as metal sulfides (see U.S. Pat. No. 4,354,937 by Hallberg). Also, in U.S. Pat. No. 4,789,478, Revis et at. used a mixed culture of Citrobacter freundii and sulfate reducing bacteria so as to precipitate heavy metal ions into sulfide form. Furthermore, in U.S. Pat. No. 4,522,723, Kauffman et at. were able to precipitate out metals into sulfide species using sulfate reducing bacteria of the genera Desulfovibro and Desulfotomaculum.
Another approach was taken by Lupton et at. in U.S. Pat. No. 5,062,956. Using anaerobic sulfate reducing bacteria, they were able to precipitate chromium as an insoluble hydroxide since the removal of sulfate causes a rise in pH. Similarly, in U.S. Pat. No. 4,519,912, Kauffman et al. were able to remove sulfate and heavy metals from aqueous solutions using mixed cultures of anaerobic bacteria of the genus Clostridium and at least one other bacteria from the genera Desulfovibrio and Desulfotomaculum. Kauffman et at. used a treatment zone to carry out the removal of water soluble species of heavy metals including selenium and sulfate ions. Analogously, in U.S. Pat. No. 4,519,913, Baldwin et al. disclosed the use of a porous matrix for retaining a population of bacterium of the genus Clostridium thereby reducing the concentration of water soluble ionic selenium species. The bacteria were grown under anaerobic conditions and definitive temperature and pH conditions.
In addition to the anaerobic methods discussed above, there have been several reports of using aerobic microorganisms to reduce the concentrations of ions in waste water. For example, in U.S. Pat. No. 3,923,597, Chakrabarty et al. used a genetically engineered species of Pseudomonas to remove mercury as a pollutant or an impurity. Also, in U.S. Pat. No. 4,468,461, Bopp was able to remove chromate from waste water using a strain of Pseudomonas fluorescens. Furthermore, in U.S. Pat. No. 4,728,427, Revis et at. were able to reduce the concentration of at least one heavy metal from an aqueous waste solution using a culture of Pseudomonas maltophilia. Moreover, in U.S. Pat. No. 4,898,827, Brierly et al. were able to remove ionic species of Ag, Cu, Pb, Cr, and Ni from waste waters using Bacillus subtilis and in the process recover these metals.
It has also been shown that metals can be recovered from aqueous solutions such as industrial waste water using several species of fungi (see U.S. Pat. No. 4,293,333 by Drobot).
In U.S. Pat. No. 4,530,763, Clyde et al. describe a method for treating waste fluids so as to remove selected chemicals with bacteria attached to porous fiber webbing. The waste fluids, containing nutrients, are drawn through the porous fiber webbing wherein ions of Cr, Ur, Fe, Ag, Pb, and V are removed from the solution.
Finally, it has been shown that manganese can be chemically removed from waste water by the addition of bisulfite to precipitate the manganese as manganese dioxide (see U.S. Pat. No. 3,349,031 by Hatch et al.).
Although all of the above-discussed methods are directed to treating waste water solutions and/or removing contaminants from waste water solutions, none are directed toward a process for removing iron and/or manganese ions from an aqueous solution by passing the aqueous solution through a porous matrix that is inoculated with metal oxidizing bacteria and is maintained under aerobic conditions. The present invention is directed to such a process.